1. Field of the Invention
This invention relates to a measuring apparatus, and more particularly, to length measuring apparatus which can measure even a minute displacement of an object using a diffraction grating.
2. Description of the Related Art
So-called grating interference measuring apparatuses in which laser light is projected onto a diffraction grating, diffracted light beams generated from the grating interfere with each other, and the amount of movement of the diffraction grating is detected from a detection signal representing the interference light utilizing the phenomenon that the intensity of the interference light periodically changes in accordance with the movement of the diffraction grating, are widely used for measuring apparatuses, production apparatuses and the like because of high accuracy in detection and their small size.
Conventional grating interference measuring apparatuses which use the following approach have been known. Light emitted from a laser light source is separated into a P-component light beam and an S-component light beam by a polarizing beam splitter, and the beams are projected onto a diffraction grating. Diffracted light beams generated by the diffraction grating are again projected onto the diffraction grating by reflecting elements, and second diffracted light beams of each incident light beam travel back along their respective initial incident optical paths and again enter the polarizing beam splitter.
.lambda./4 plates are provided in the respective optical paths. Since the respective light beams pass through the corresponding .lambda./4 plates twice, the P-component light beam and the S-component light beam are converted into an S-component light beam and a P-component light beam, respectively, when reaching the polarizing beam splitter after being diffracted twice. Accordingly, the two light beams returning to the polarizing beam splitter are combined by the polarizing beam splitter, and emanate in a direction different from the direction of the laser light source. The amount of movement of the diffraction grating is detected by making the two emanating light beams interfere with each other by being passed through analyzers, and detecting the interfered light by photosensors.
In the above-described conventional approach, a single polarizing beam splitter separates light into two light beams and combines the two light beams. In order to make the direction of the light beams emanating from the polarizing beam splitter to the photosensors after being diffracted twice differ from the incident direction of light from the light source to the polarizing beam splitter, /4 plates are provided in the optical paths.